X-ray tube voltage control circuit



Jan. 20, 1959 ooooo-m-n-o- J. FRANSEN 2,870,340

X-RAY TUBE 'VOLTAGE CONTROL CIRCUIT Filed Nov. 2, 1955 Ra T2 FIGJ Volta g:

Stabilizer t: Timer AAA FIG-2 INVENTOR JdcoAuJ Hansen United States Patent X-RAY TUBE VOLTAGE CONTROL CIRCUIT Jacobus Fransen, Eindhoven, Netherlands, assignor, by

mesne assignments, to North American Philips Conipany, Inc., New York, N. Y., a corporation of Delaware Application November 2, 1955, Serial No. 544,514

Claims priority, application Netherlands November 3, 1954 5 Claims. (Cl. 250-103) The present invention relates to an X-ray tube voltage control circuit.

X-ray tubes are usually accompanied by one or more regulating devices, for example, a device for selecting the voltage between the anode and the cathode of the X-ray tube (tube voltage) and also a regulating device for adjusting the current which. serves to heat the incandescent cathode of the X-ray tube. The regulating device permits the control of the electron emission in the X-ray tube (tube current).

In certain cases, steps are taken to make the tube current independent of the position occupied by the voltage selector. These steps afford the advantage that the different load values may be adjusted more easily in taking X-ray photographs. When the required tube voltage as a function of the object to be photographed has been determined, it is possible to ascertain how much the product of tube current and exposure time, hence the charge, must be to obtain a determined blackening of the photographic plate. The charge adjusted by the current regulator and the time selector may be indicated on a reading scale which is usually calibrated in units representing the product of milliamperes and seconds. If the tube current is dependent upon the tube voltage, a difierent milliampere-scale is required for any tube voltage to be selected. If, however, the tube current does not vary as a result of variation in voltage, the same milliampere-scale may serve for any voltage selected.

Several means may be utilized to make the tube current independent of the position of the voltage selector. It is possible, for example, to include a variable resistor in the heating-current circuit of the X-ray tube and to couple its regulating member to the regulating member which serves to select the tube voltage. As an alternative, for the same purpose, use is made of a transformer, the primary voltage of which is controlled by means of an operating member coupled to the voltage selector and the secondary winding of which is included in the heatingcurrent circuit. The eliect of such steps is not very accurate as a result of the fact that the tube voltage is determined not only by the position of the voltage selector, but also by the voltage drop across the different resistances, which voltage drop varies with the current strength. Consequently, they can lead to perfect correction only for a predetermined value of the tube current.

For varying the tube voltage, use is made of a transformer of adjustable transformation ratio. The voltage drop resulting from the resistance in the primary thereof varies not only with the tube current, but also with the transformation ratio.

The X-ray tube is usually accompanied by a graph showing a plurality of curves each corresponding to a predetermined current and indicating the tube voltage asa function of the transformation ratio of the regulating transformer. The graph indicates the position which the voltage selector must occupy to produce a determined tube voltage for a given tube current. As an alternative, use may be made of a table showing the re- 2,870,340 Patented Jan. 20,

ice

'measurements with the X-ray tube for each positionof the voltage selector, in each case for a series of values of the tube current. At the place of installation of the tube, the resistance is definitely adjusted to the-value on which the graph is based.

An object of the invention is to facilitate the production of the voltage graph and the adjustment of the tube voltage. For this purpose, the X raytube control circuit of the present invention comprises means for insuring that-the voltage drop for any position of the voltage selector is constant for a given value of the tube current. In this case the lines of the voltage graph are straight and the tube voltage for a series of values of the tube current is to be determined in only one position of the voltage selector.

The person in charge of the adjustment ofthe different load values for an X-ray circuit in accordance with the invention will sooner be able to operate the voltage selector without consulting the graph, since the nominal value associated with an arbitrary position of the voltage selector need onlybe decreased (or increased) by an amount equal for all positions to obtain the value of the actual tube voltage, which amount is otherwise also dependent upon the tube current.

According to the invention, the selector for the tube voltage is coupled to a variable resistor included in the supply circuit of the X-ray tube, so that the impedance in this circuit, and hence the voltage drop, varies when the voltage varies. When in a manner which will be explained hereinafter, the resistance value required of said resistor is calculated for the various positions of the voltage selector to maintain constancy of the voltage drop, a non-linear relation between the impedance and the transformation ratio of the regulating transformer is found. However, it appears that no error of ,consequence arises when use is made of a resistor, the regulating steps of which are proportional to those of the transformation ratio, provided that care is taken to 'insure that the voltage drop is substantially equal at least at the boundaries of the regulating range of the tube voltage. v

The control circuit of the present invention may alternatively be designed so that the tube voltage is no longer dependent upon the position of the current regulator. In this case it is no longer necessary to consult a graph, or a table, or to keep in mind a series of correction values, in order to adjust the tubevoltage; the reading scale associated with the member for operating the voltage selector may be provided with a number indicating directly the value of the tube voltage for eac position of said member. i H

For this purpose, an auxiliary voltage is added to the supply voltage for the X-ray tube. The auxiliary voltage is constant for any transformation ratio of the regulatingtransformer, but varies with the tube current. Consequently, it may be chosen by theoperating member of the current regulator.

voltage, instead of being determined by the tube current itself, is determined by the position of the regulating member of the tube current and is no longer of importance.

Finally, the tube current tion of load, so that the tube current decreases with an regulator mayv be coupledin-lknown manner to a timing device for selecting the 'dura-il increase in the duration of load, and conversely. It is thus possible always to obtain automatically the optimum load on the tube and to avoid overload of the tube.

In order that the invention may be readily carried into efiect, it will now be described, by way of example, with reference to the accompanying drawing, in which:

" Fig. 1 is a schematic diagram of an equivalent circuit of an X-ray tube voltage control circuit; and

' Fig. 2 is a schematic diagram of an embodiment of the X-ray tube voltage control circuit of the present invention. v The cause of the vo tage drop and the manner in which its value may be determined, may be observed bv reference to Fig. 1. In Fig.1, the regulating transformer, which is of the autotransformer type, and the high-tension transformer, are indicated by T and T respectively. The variable transformation ratio of the autotransformer is represented by N and the constant transformation ratio of the high-tension transformer is represented by N When the voltage of the power supply. w ich may be regarded as constant, is represented by E then the tube voltage which occurs when the tube current has the value zero (no-load voltage) is E =N N E,,.

Theresistance of the primary circuit in the regulating transformer is represented by a concentrated resistor R in Fig. l. the remaining resistance in the primary circuit of the high-tension transformer being represented therein by a concentrated resistor R,,. The resistance of the secondary circuit of the high-tension transformer, which is connected in series with the X-ray tube and includ s the resistance of any rectifying tubes present, is indicated by a concentrated resistor R, in Fig. l.

The X-ray tube voltage, which is obtained when the Xray tube current is switched on, is

wherein E is the drop in voltage due to the resistances R,,, R,, and R In other words, E is the difference between the no-load tube voltage and the loaded tube voltage.

When the tube current has a value I the voltage drop may be represented by This comprises two components I (R -l-R N and IR N N The first componentI(R -l-R,,N is not dependent upon the transformation ratio N and the second component IRgN N varies quadratically with N It is to be noted that R actually is slightly dependent upon N However, by suitable adjustment of the resistance steps, erroneous resultant values are avoided.

' The variation of the component lR N N of the voltage drop is neutralized by an additional variable resistor in the primary supply circuit of the high-tension transformer, the regulating member of which is coupled to that of the regulating transformer. Consequently, the value R varies when N varies. In the equation the value of E becomes independent of the adjustment of the voltage selector if IR N- -i-IR N N =C C represents a constant value. Thus, if

sequently the term E;,-(IR -l-IR N j-IR N N must be constant. When the expression is substituted for R,,, the desired operating condition requires that E;,IR must provide a constant value and hence that the auxiliary voltage must vary in proportion with current.

Fig. 2 is a schematic diagram of an embodiment of the X-ray tube voltage control circuit of the present invention. A transformer 1, which is of the autotransformer type, is connected to the power supply terminals via a variable resistor 3. The resistor 3 serves to bring the impedance of the primary circuit of the regulating transformer 1 to that value which is used in calibrating the device or in providing the voltage graph. Said primary circuit also includes a regulator 4 which can connect one of the power supply lines 2 to different tapping points on the winding of the transformer 1. By means of the regu lator 4, it is possible for a voltmeter 5, provided between two fixed points of the transformer Winding, to be always adjusted to a prescribed indication irrespective of local variations in the power supply voltage.

The heating current for the cathode of the X-ray tube 6 is supplied by a heating-current transformer 7. Since several methods are known for maintaining the tube current constant, upon variation of the tube voltage, it is not necessary to refer thereto in detail. For the sake of completeness, a variable resistor 8 which serves to maintain the tube current constant, is shown in the primary circuit of the heating-current transformer 7. The regulating member of the resistor 8 is coupled to a voltage selector 13.

The heating current circuit also includes a voltage stabilizer 9 and a variable resistor tube current regulator 10. The resistor 16 serves to control the tube current.

The AC output in the secondary winding of a high tension transformer 12 is rectified by a rectifier bridge 14 and is supplied to the X-ray tube 6.

The regulating transformer 1 supplies the supply volage to the high-tension transformer 12. The primary winding of the high tension transformer 12 may be connected, by way of the voltage selector 3, to different tapping points 15 on the winding of the regulating transformer I.

The primary circuit of high tension transformer 12 includes a variable resistor 18, the regulating member of which is coupled to the voltage selector 13. The selector 13, the resistor 18 and the resistor 8 are coupled together mechanically, or in another manner, said elements being shown connected together by a dot-and-dash line.

It is intended that, when the position of the voltage selector 13 is varied, the value of the resistor 18 be varied in such a manner that the voltage drop is not affected by this variation in position. Although it has been determined that for this purpose the resistance value of the resistor 18 must vary quadratically with the transformation ratio of transformer 1, it has been found that for the comparatively small regulating range commonly used in practice, a linear variation provides sutiiciently accurate approximation. The maximum value of N to which the voltage selector 13 may be adjusted, is usually not higher than thrice the minimum value. For this regulating range, the maximum deviation which occurs when the parabolic curve showing the resistor R, as a function of N is substituted by a straight line connecting the extreme points of this curve (hence the points associated with the maximum and minimum values of N rendered possible by the voltage regulator) is not more than 20%. The voltage drop is usually maintained below a reasonable limit by suitable choice of the various resistance values of the circuit and will seldom be more than 25% of the no-load voltage. substituting a resistor varying linearly with N for a resistor varying with N in the theoretically correct manner,

In the most unfavorable case of causes only an error of at most 5% and may therefore be regarded as coming within the scope of the invention.

The core of the regulating transformer 1 carries a separate auxiliary winding 16. The voltage induced in the winding 16 serves to make the voltage drop independent of the position of the regulator for the tube current. Due to the inclusion of the winding 16 in one of the supply lines to the high-tension transformer 12, the voltage induced in said winding constitutes a contribution to the supply voltage of said transformer.

As determined, the auxiliary voltage E produced in the winding 16 must vary with current. In order to achieve this, provision is made of a regulating member 17 which may be connected to different tapping points on the auxiliary winding 16 and which is couped to the regulating member of the resistor as shown by a dot-and-dash line. The regulating steps of the auxiliary winding 16 may best be determined empirically.

The supply current of the high-tension transformer 12 (at the beginning and at the end of the duration of load) is switched on and off by means of an electromagnetically operated switch 19. The energizing current of the switch 19 is controlled by a timer 11. If desired, the timer 11 may be coupled in known manner to the regulating member of the resistor 10 (as shown by a dot-and-dash line) so that either the X-ray tube is prevented from being loaded more strongly than is permissible for the chosen duration of load, or an optimum load on the X-ray tube is always obtained. I

The variable resistor 18 may alternatively be included in the primary circuit, instead of in the secondary circuit, of the autotrausformer 1. The condition that the regulating characteristic of the resistor theoretically has to satisfy in such a case may be readily determined from the foregoing equations. In this case, the auxiliary voltage of the auxiliary winding 16 must vary in proportion with the tube current.

It is to be understood that the invention is not limited to the details disclosed but includes all such variations and modifications as fall within the spirit of the invention and the scope of the appended claims.

Having thus set forth the nature of my invention, what I claim is:

1. An X-ray tube voltage control circuit, comprising an X-ray tube having an anode and a cathode, a source of power supply, transformer means having a variabletransformation ratio and being coupled between said supply source and saidanode and cathode of said tube, a selective member for varying the transformation ratio] of said transformer means, variable impedance means having an impedance regulating member, said impedance means being interposed in the supply circuit of the anode between said supply source and said tube, and means for coupling said regulating member to said selective member, said impedance means at a first given position of said selector and regulating member having a value rendering the total voltage drop resulting from the load on the X-ray tube equal to that occurring at a second given position of said selector and regulating member.

2. An X-ray tube voltage control circuit, comprising an X-ray tube having an anode and a cathode, a source of power supply, transformer means having a variable transformation ratio and being coupled between said supply source and said anode and cathode of said tube, a selective member for varying the transformation ratio of said transformer means, variable impedance means having an: impedance regulating member, said impedance means being interposed in the supply circuit of the anode between said transformer means and said tube and being variable in steps proportional to those of said transformation ratio, and means for coupling said regulating member to said selective member, said impedance means at a first given position of said selector and regulating member having a value rendering the total voltage drop resulting from the load on the X-ray tube equal to that occurring at a second given position of said selector and regulating member.

3. An X-ray tube voltage control circuit, comprising an X-ray tube having an anode and a cathode, a source of power supply, transformer means having a variable transformation ratio and being coupled between said supply source and said anode and cathode of said tube, a selective member for varying the transformation ratio of said transformer means, variable impedance means hav ing an impedance regulating member, said impedance means being interposed in the supply circuit of the anode between said transformer means and said tube and bzing variable in steps having intermediate vaues which are a quadra ic function of said transformation ratio thereby to provide a substantially constant voltage drop through its entire regulating range, and means for coupling said regu ating member to said selective m mber, said imped nce means at a fi st given positi-n of aid sele'tor and regu at ng mem er havin a va'ue rendering the total voltage drop resu ting from the load on he X-ray tu e equal to that occurr n at a second given position of said selector and regu a ing member.

4. An X-rav tu e volta e contr l circuit. com rising an X-rav tube havin an anode and a cath de. a s urce of power su uly, transformer means h in a v riab e tr nsform tion ratio and being cou ed between said su lv source and sa d anode and cathode of said tu e, a first selecti e m m er for varving the transformati n ratio of said transformer means. variable im edance means having an imped nce re ulating member, said im ed nce means bein inter osed in the suoulv circuit of the anode between said tr nsf rmer means and said tube. means for derivin a variable auxiliary volta e fr m said sunplV source, said last-mentioned means including a second selective member for varving said auxiliary voltage, means for coupling said regulating member to said first selective member, means for supplying a current to the cathode of said tube including further variable impedance means interposed between said supply source and the said cathode and having an impedance regulating member. and means for coupling the regulating member of said further variable impedance means to said second selective member whereby said auxiliary voltage varies with the current in the X-ray tube and differs by a substantially constant amount from the total voltage drop resulting from the load on said tube-regardless of the position of the regulating member of said further variable impedance means.

5. An X-ray tube voltage control circuit, comprising an X-ray tube having an anode and a cathode, a source of power supply, transformer means having a variable transformation ratio and being coupled between said supply source and said anode and cathode of said tube, a first selective member for varying the transformation ratio of said transformer means, variable impedance means having an impedance regulating member, said impedance means being interposed in the supply circuit of the anode between said transformer means and said tube, means for deriving a variable auxiliary voltage from said supply source, said last-mentioned means including a second selective member for varying said auxiliary voltage, means for coupling said regulating member to said first selective member, means for supplying a current to the cathode of said tube including further variable impedance means interposed between said supply source and the said cathode and having an impedance regulating member, means for coupling the regulating member of said further variable impedance means to said second selective member,

timing means, and means for coupling said timing means I to the regulating member of said further variable'impedance means whereby the current in the X-ray tube decreases with increasing duration of load on said tube and increases with decreasing duration of load on the said tube.

No references cited. 

